Dunhuang Block Research Articles

Subduction retreat recorded in the southern Beishan orogenic collage of the Central Asian Orogenic Belt: New evidence from deep seismic reflection profiling

The crustal and upper mantle structure of the Beishan orogenic collage, which serves as the southern part of the middle Central Asian Orogenic Belt, provides crucial insights into the history of the multiple openings and closings of the Paleo-Asian Ocean during the Paleozoic. There is considerable dispute over the eventual closure position, timing, and subduction polarity of the Paleo-Asian Ocean, particularly in the southern Beishan orogenic collage. The main cause of these controversies is the lack of a high-resolution lithospheric structure in this area. In this study, we first present a 140-km-long, high-resolution seismic reflection profile taken across the northern Dunhuang Block and the southern Beishan orogenic collage. The seismic imaging provides new constraints on the structure of the lower crust, the Moho, and the upper mantle beneath the southernmost Central Asian Orogenic Belt. A subhorizontal reflector in the middle crust, two sets of north-dipping reflectors from the lower crust to the upper mantle, and several south-dipping reflectors in the upper crust in the northern part of the profile were imaged. Based on our study and other geological, chronological, and geophysical data, we propose that the two sequences of north-dipping reflectors from the lower crust to the upper mantle represent two stages of north-dipping subduction of the southern branch of the Paleo-Asian Ocean. The first stage in the southern Beishan orogenic collage is Late Silurian−Early Devonian, and the second stage is late Carboniferous−Early Permian. The two-stage subduction process gave rise to the Huaniushan arc and the subsequent Shibanshan arc, respectively. These findings provide new constraints on the controversial subduction polarity and the multistage amalgamation of the microcontinental blocks and arcs in the southern Central Asian Orogenic Belt.

Devonian Andean‐type convergence in the southern Dunhuang block (NW China): Petro‐structural, metamorphic P–T and geochronological constraints

AbstractArchean to Palaeoproterozoic basement rocks exposed in the Dunhuang block in NW China were affected by Palaeozoic crustal reworking, as constrained by previous zircon U–Pb geochronological investigations. However, relationships between the Palaeozoic metamorphic ages, P–T evolution and deformational history of the region remain ambiguous. In order to address this issue, P–T–t–D paths of paragneisses from the basement of the Hongliuxia belt in the southern Dunhuang block were investigated. Inclusions in garnet and kyanite from the paragneisses are considered as vestiges of Palaeozoic M1 metamorphism corresponding to initiation of the prograde evolution. The earliest continuous metamorphic fabric is an originally steep N–S striking foliation S2. This fabric was reworked by vertical folds F3 associated with the development of a ubiquitous steep, mainly south‐dipping, E‐W striking axial planar foliation S3. The S2 foliation in paragneisses is mainly associated with Grt–St–Ky–Sil–Bt–Ms–Pl–Qz–Rt assemblages in samples from the western domain and with Grt–Ky–Sil–Bt–Kfs–Pl–Qz–Rt assemblages in samples from the northeastern domain of the Hongliuxia belt. The S3 foliation is associated with Grt–Sil–St–Bt–Ms–Pl–Qz–Ilm assemblages in the western domain and with Grt–Sil–Bt–Ms–Pl–Qz–Kfs–Ilm assemblages in the northeastern domain, followed by growth of chlorite in both domains. Early prograde stage (M1) from 4.0–6.5 kbar and 540–560°C to metamorphic peak (M2a) at 9–10 kbar and ~650–675°C is mainly recorded by paragneisses from the western domain. Subsequent decompression is initially accompanied by heating (M2b) constrained to 6.5–7 kbar and 675–710°C in the western domain, and to 6–6.5 kbar and ~730°C in the northeastern domain, followed by cooling (M3) through 4–6.5 kbar and 550–650°C till late chloritization (late M3). In situ U–Pb dating of monazite combined with monazite trace‐element compositions suggests that prograde evolution (M1) most likely started at c. 406 Ma, peak‐P conditions (M2a) were reached at 400–394 Ma, decompression associated with heating (M2b) took place at 393–391 Ma, and cooling (M3) during exhumation probably lasted from 380 to 354 Ma. The prograde metamorphism probably reflects burial during underthrusting of neighbouring continental basement (the Alxa block or an equivalent) below the Dunhuang block. This event culminated in pure shear thickening (D2a) of the whole supra‐subduction margin followed by minor heating and exhumation (D2b). The D3‐M3 event is interpreted as reflecting exhumation during orthogonal shortening of the system, possibly in response to an independent orogenic cycle. Combined with the available regional data, this study reveals the existence of a complex tectono‐metamorphic evolution for the Dunhuang block characterized by two distinct orogenic phases with (i) the thickening of a previously thinned arc‐back‐arc crust recorded in the northern and central belts at 420–410 Ma in the pro‐wedge side of the active margin (Sanweishan phase), followed by (ii) the 410–390 Ma thickening in the retro‐wedge side (Hongliuxia phase). Such a tectonic evolution of the whole Dunhuang block resembles Andean‐type migration of crustal thickening from the convergent front to hinterlands. The D3‐M3 event, potentially responsible for the juxtaposition of rocks from different geological occurrences and depths, is seemingly independent from this Andean‐type orogenic cycle.

Mineralogical characteristics and fluid inclusions of the Dashuixiabei crystalline graphite deposit in the Dunhuang block

AbstractGraphite is a strategic mineral resource of the world. The Dashuixiabei (DSXB) crystalline graphite deposit is located in the Dongbatu area in the Altyn crystalline graphite metallogenic belt. This study presents data from x‐ray diffraction (XRD), scanning electron microscopy (SEM), laser Raman spectroscopy, and fluid inclusions analysis conducted on the DSXB crystalline graphite deposit. The graphite is flake shaped, with a diameter of 0.1–0.4 mm. A group of extremely complete cleavage was observed under SEM, and the overall distribution was a flake. The C content in graphite is greater than 90 wt%, followed by N, O, and so on. The results of XRD analysis show that the lattice parameters of the DSXB crystalline graphite is a = 0.246–0.2465 nm, c = 0.6714–0.6715 nm, and V = 0.03519–0.03532 nm3. The degree of graphitization ranges from 0.8490 to 0.8519, indicating that the graphite crystal structure is relatively intact, the average metamorphic temperature is 552°C, and the content of 3R polytypes ranges from 13.16% to 13.25%. It is concluded that the metamorphic rocks of the DSXB crystalline graphite deposit have undergone medium‐grade metamorphism. The Raman spectral peaks show that DSXB crystalline graphite has a high degree of crystallinity. The DSXB crystalline graphite ore is dominated by hydrothermal fluids with medium‐low temperature and medium‐low salinity, and the vapor phase of the fluid inclusions is composed of CH4. There may be a superposition of the main and secondary mineralization stages. The crystalline graphite from the DSXB deposit shows high quality and stable ore quality, which has good metallogenic conditions and development and utilization prospects.

Magmatism and tectonic evolution at an active continental margin: A case study of the Dunhuang Block

Magmatism and tectonic evolution at an active continental margin: A case study of the Dunhuang Block

Paleoproterozoic crustal evolution of the Tarim Craton, NW China: Constraints from geochronology and geochemistry of orthogneisses and granitic veins in the Xingdi region of the Quruqtagh Block

The Archean–Paleoproterozoic basement rocks, exposed in the Xingdi region of the Quruqtagh Block, provide a great opportunity to characterize the early crustal evolution of the Tarim Craton and reconstruct the Columbia supercontinent. Here we report zircon U–Pb ages, Lu–Hf isotope compositions, and bulk-rock geochemical data of orthogneisses and granitic veins from the Xingdi region of the Quruqtagh Block in the northeastern margin of the Tarim Craton. The orthogneisses formed in the late Neoarchean to early Paleoproterozoic (2504 ± 33 Ma, 2419 ± 24 Ma and 2450 ± 27 Ma), and were metamorphosed in the late Paleoproterozoic (1789 ± 43 Ma and 1748 ± 35 Ma). The orthogneisses have relatively high Sr (312–710 ppm) contents, and low Y (5.3–13.8 ppm) and Yb (0.46–1.10 ppm) contents, resulting in high Sr/Y (32–118) and La/Yb (17–64) ratios that are characteristics of adakites. The relatively high Mg# (49–55) and depleted zircon Hf isotope characteristics (positive εHf(t), with TDM2 of ca. 2.5–2.8 Ga) indicate that the orthogneisses crystallized from partial melting of garnet-bearing amphibolite in a subduction environment. The granitic veins that intrude the orthogneisses formed during the late Paleoproterozoic (1760 ± 25 Ma), which is roughly contemporaneous with the metamorphic ages of the orthogneisses; the granitic veins are characterized by extremely high SiO2 (>80 %), enrichments in Rb, Ba and U, and depletions in Nb and Ta. It is inferred that the granitic veins formed by in situ anatexis of the orthogneisses during the late Paleoproterozoic. Based on the zircon ages with Hf isotope compositions, it is evident that the continental crust in the Xingdi region of the Tarim Craton largely formed between 2.5 and 2.8 Ga and was reworked at ca. 1.8 Ga; this evolutionary history is similar to that of the Dunhuang Block. Comparison of the Paleoproterozoic tectono-thermal events in the different terranes of the Tarim Craton, it is suggested that the Tarim Craton is composed of discrete terranes that detached from different ancient cratonic nuclei prior to the late Paleoproterozoic, and assembled within the Columbia supercontinent respectively.

Neoarchaean DTTGs from the Dunhuang Block, Tarim Craton: insights into petrogenesis and crust–mantle interactions

ABSTRACT Earth’s first continental crust is formed by Archaean and mainly consisted of tonalite–trondhjemite–granodiorite with a small amount of diorites (DTTGs), which has an essential role in probing early crust–mantle dynamic regime and in understanding the formation mechanism of continental crust. Here, we present zircon U‒Pb dating and Lu‒Hf isotopes, whole-rock geochemistry, and petrography on DTTGs rocks in the Dunhuang Block. Three episodes of DTTGs were emplaced circa 2.67 Ga, 2.60 Ga, and 2.50 Ga. The circa 2.67 Ga TTGs exhibit high SiO2 contents (68.14–71.70 wt%), low MgO contents (0.65–1.31 wt%), and high ratios of (La/Yb)N (146 on average), with their enriched Nd-Hf isotopes [ƐHf (t) = -5.48–3.19 and ƐNd (t) = -5.77–0.53], indicating origination from partial melting of amphibolites at thickened lower crust. In contrast, the circa 2.60 Ga transitional TTGs exhibit relatively high MgO contents (2.80–3.39 wt%), flat REE (Rare earth element) patterns with moderate ratios of (La/Yb)N (20.49 on average), and dispersed Nd-Hf isotopes [ƐHf (t) = -5.48–3.19 and ƐNd (t)= −3.99–3.08]. Accordingly, circa 2.60 Ga transitional TTGs melts were produced by partial melting of the shallower crust induced by mantle-derived magma upwelling. The circa 2.50 Ga diorites exhibit low SiO2 (55.72–59.11 wt%) but high MgO (3.51–4.52 wt%) contents with positive Nd-Hf isotopes [ƐHf (t) = -0.16–4.17 and ƐNd (t) = 2.00–4.45], suggesting that they originated from partial melting of mantle wedges metasomatized by fluid from subduction slabs. Combined with the detailed petrogenetic studies and crustal thickness variation, we conclude that the complex crust–mantle interaction may be an essential reason for the Neoarchaean diversity of DTTGs from the Dunhuang Block, which experienced prolonged arc accretion before Neoarchaean, followed by delamination between 2.67 and 2.60 Ga and subsequently transitioned to subduction.

Long-lived subduction retreating led to continental rifting along the northern Gondwana: Insights from Devonian igneous rocks and ophiolite in the Beishan orogenic collage

The Beishan orogenic collage is an ideal setting for investigating the temporal and spatial relationships between extensional tectonics in the Central Asian Orogenic Belt and the rifting of Gondwana. This study presents new geochemical and geochronological data for Devonian igneous rocks and the Zhangfangshan ophiolitic mélange in the southern Beishan region. The Zhangfangshan ophiolitic mélange contains a Late Devonian gabbro block (ca. 369 Ma) with extremely depleted zircon Hf isotopic compositions (εHf[t] = +16.5 to +22.4). The Lucaogou diorites (ca. 367 Ma) have low MgO and compatible element contents, low Sr/Y and La/Yb ratios, and positive zircon εHf(t) values, indicative of derivation by dehydration melting of Mesoproterozoic mafic igneous rocks in a high-temperature and medium- to low-pressure environment. The Lucaogou K-feldspar granites (ca. 374 Ma) and Dundunshan silicic volcanic rocks (ca. 375 Ma) have A-type granite affinities, and were probably derived from magmas formed by mixing between Neoproterozoic orthogneiss-derived melts and depleted mantle-derived mafic magmas in a high-temperature shallow-crustal environment. Compiled geochronological data for subduction-related magmatism and supra-subduction zone-type ophiolites are consistent with protracted subduction–accretion until the early Carboniferous along the northern margin of the Dunhuang Block. Slab retreat in this long-lived subduction zone led to the generation of oceanic lithosphere in the southern Beishan region. Furthermore, Devonian extensional tectonics in the southern Beishan area coincided with the opening of the Paleo-Tethys Ocean, indicating that protracted subduction retreat caused the separation of the Dunhuang Block from East Gondwana.

Devonian continental arc magmatism in the southern Central Asian Orogenic Belt: Evidence from the Dunhuang Block, NW China

Reconstruction of periods and sources of continental arc magmatism provides critical insights for deciphering the thermal evolution and geodynamics of the continental lithosphere and unveiling subduction-related episodes of accretion and orogeny. The Dunhuang Block, located in the southern Central Asian Orogenic Belt, is built upon a Precambrian metamorphic crystalline basement that is intruded by numerous Paleozoic granitoids. In this paper, we present new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Pb-Hf isotopes from the Hanxiakouzi and Shuangta granodiorite plutons outcropped in the central and northern Dunhuang Block, respectively. The U-Pb zircon dating indicate that the Hanxiakouzi and Shuangta granodiorites were emplaced at ca. 366–363 Ma, i.e., in the late Devonian. Geochemically, the Hanxiakouzi granodiorites are medium- to high-K, I-type granites possessing high Sr and Sr/Y, i.e., similar to adakites. The Hanxiakouzi granodiorites show negative εNd(t) values (−8.2 to −6.6), significant positive εHf(t) values (+0.7 to +8.1), and evolved Pb isotope features suggesting their derivation by melting of subducted terrigenous sediments, interaction of melts with mantle peridotite, and assimilation of ancient crust during magma ascent. The Shuangta granodiorites belong to high-K calc-alkaline I-type granites with medium Sr and low Sr/Y. The Shuangta granodiorites yield negative εNd(t) values (−2.9 to −3.1) and negative to positive zircon εHf(t) values (−1.8 to +3.6), suggesting their formation by partial melting of ancient mafic crust with addition of minor mantle-derived melt and subducted terrigenous sediments. In general, Hf-in-zircon isotopes characteristics of the late Paleozoic granitic rocks in the Dunhuang Block indicate that the southern Dunhuang area carries more ancient crust materials than the central and northern Dunhuang areas, indicating a lateral heterogeneity of the middle Paleozoic crust. Conclusively, the middle Paleozoic granitoids of the Dunhuang Block were emplaced at an active continental margin of the southern Paleo-Asian Ocean.

Cambrian-Early Ordovician intrusive bodies in Dunhuang Block: Records of Paleo-Asian Ocean subduction and implications for regional Early Paleozoic tectonic evolution

一直以来，敦煌地块缺少1.6~0.46Ga的地质记录，从而严重制约了对该地块在新元古代和早古生代期间地质构造演化的全面认识。通过15万区域地质调查，本次工作在敦煌地块东北缘新发现了寒武纪-早奥陶世小宛山岩体、截山子岩体和小宛南岩体等多个中酸性侵入体，测得其LA-ICP-MS锆石U-Pb年龄分别为517±3Ma、480±3Ma和473±3Ma，由此厘定出敦煌地块目前古生代最古老的侵入岩体及早奥陶世侵入岩体。通过这些岩体的岩石岩相学、岩石地球化学特征、岩石成因及大地构造环境分析，表明它们属于与洋壳俯冲消减作用有关的活动陆缘环境下形成的富钠质I型花岗岩。其中，小宛山花岗闪长质岩体是在低压低温条件下由玄武质地幔楔部分熔融而成，同时受到俯冲流体的强烈交代；截山子岩体和小宛南岩体则是在高压低温富水条件下，由新生镁铁质洋壳发生部分熔融并受到地幔楔强烈混染而形成，虽然它们均属于（类） O型埃达克岩，但其部分熔融的压力及其残留矿物组合不同。上述研究揭示敦煌地块北缘早古生代517Ma就存在俯冲作用，且至少持续了44Myr。结合区域地质资料和以往研究成果，敦煌地块北缘早古生代洋陆转换过程可分为三个阶段：（1）晚震旦世-早寒武世（574~518Ma），敦煌地块北缘被动陆缘演化阶段；（2）寒武纪第二世-早奥陶世（517~471Ma），敦煌地块北缘活动陆缘演化阶段，期间，古亚洲洋南支洋分别向敦煌地块和石板山地块/马鬃山地块发生双向俯冲消减；（3）中奥陶世-早泥盆世（464~412Ma），敦煌地块与石板山地块/马鬃山地块碰撞造山阶段，期间古亚洲洋南支洋闭合。

Geochemical characteristics and genetic implications of the Dashuixiabei crystalline graphite deposit in the Dunhuang Block

Geochemical characteristics and genetic implications of the Dashuixiabei crystalline graphite deposit in the Dunhuang Block

Early Paleozoic intermediate igneous rocks in the Dunhuang Block, southern Central Asian Orogenic Belt: geochronology, petrogenesis, and implications for tectonic evolution

Late Ordovician–Early Silurian intermediate igneous rocks have been detected in the Dunhuang Block, NW China. These igneous rocks help to constrain the tectonic evolution of the Dunhuang Block and the southern Central Asian Orogenic Belt in the early Paleozoic. We carried out zircon U–Pb dating, whole-rock geochemistry and zircon Hf isotope analyses for the early Paleozoic gneissic diorites from the Sanweishan and northern Huangshuihou areas in the Dunhuang Block. The zircon U–Pb geochronology shows that the emplacement ages of the gneissic dioritic rocks were c. 443–440 Ma. The dioritic rocks show varying SiO 2 (48.1–63.1 wt%) and MgO (1.87–3.52 wt%) contents, with high Mg# (46–52) values and negative Eu anomalies (Eu/Eu* = 0.62–0.94). Zircons in the gneissic diorites from the northern Huangshuigou and Sanweishan areas yield variable ε Hf ( t ) values of −4.4 to +10.4 and −8.6 to −6.2, respectively, coupled with low Rb/Sr (0.04–0.34), Rb/Ba (0.06–0.31) and Al 2 O 3 /(MgO + FeO T ) (0.72–1.47) ratios, indicating that they were predominately generated by disequilibrium melting of the juvenile crust and mixed with ancient crustal and minor mantle melts. In conjunction with previously published data, we propose that the early Paleozoic gneissic dioritic rocks were formed in a continental margin arc setting related to the southward subduction of the Palaeo-Asian Ocean. Our findings suggest that the Dunhuang Block was modified and reactivated by the Paleozoic orogenic events related to the evolution of the Palaeo-Asian Ocean, resulting in extensive Paleozoic magmatism–metamorphism over the whole Dunhuang area. Supplementary material: Analytical methods, U-Pb geochronological data, zircon trace element data, zircon Lu-Hf isotopic compositions, and geochemical data for analysed samples, and additional figures are available at https://doi.org/10.6084/m9.figshare.c.6174601

P–T–t–D records of Early Palaeozoic Andean‐type shortening of a hot active margin: The Dunhuang block in NW China

AbstractHigh‐pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra‐subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–time‐deformation (P–T–t–D) paths. In the current study, P–T–t–D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N‐S trending sub‐vertical fabric (S2) preserved in low‐strain domains and (3) upright folds (F3) associated with a ubiquitous steep E‐W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8–11 kbar and ~850–900°C during crustal melting (M2) followed by partial re‐equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re‐equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P–T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure–temperature (P–T) evolution M3 suggests formation of a mélange‐like association during the late stages of orogeny. The high‐pressure event D1‐M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and thinned pre‐orogenic crust. The high‐temperature (HT) re‐equilibration D2‐M2 is interpreted as a result of Mid‐Devonian shortening of the previously thickened crust, possibly due to ‘Andean‐type’ underthrusting. The D3‐M3 event reflects Late Devonian supra‐subduction shortening and continuous erosion of the sub‐crustal lithosphere. This tectono‐metamorphic sequence of events is explained by polyphased Andean‐type deformation of a ‘Cascadia‐type’ active margin, which corresponds to a supra‐subduction tectonic switching paradigm.

Decoupling of metamorphic zircon U-Pb ages and P-T paths in the Dunhuang metamorphic complex, northwestern China

U-Pb dating of metamorphic zircon is commonly used to determine the timing of high-grade metamorphism, but linking zircon U-Pb ages to metamorphic P-T paths can be challenging, especially in metamorphic terranes where multiple metamorphic events result in various degrees of re-equilibration and complex metamorphic zircon growth/recrystallization. Here we carried out a detailed P-T path study, as well as zircon/titanite/apatite U-Pb dating and mineral Sm-Nd isochron dating, for garnet amphibolites from the Dunhuang Block, northwestern China, where late Paleoproterozoic metamorphism was extensively overprinted by a Paleozoic tectonothermal event and the P-T conditions of the earlier event remain elusive. Petrographic observation, phase equilibria modeling and geothermobarometry show that the garnet amphibolites share a clockwise P-T path, from 675 – 689 °C/10.0 – 11.6 kbar to 780 – 807 °C/9.9 – 11.8 kbar for the prograde to peak metamorphism, followed by significant cooling and decompression to 612 – 689 °C/4.1 – 6.0 kbar, indicating an orogenic setting. U-Pb dating of metamorphic zircons yields two groups of upper intercept ages of ca. 1.82 – 1.85 and 1.95 Ga. The ca. 1.82 – 1.85 Ga metamorphic zircons have flat HREE patterns and weak Eu* anomalies, suggesting zircon growth when abundant garnet but little plagioclase was present, consistent with high-pressure metamorphism. In contrast, the ca. 1.95 Ga zircons have steep HREE and negative Eu anomalies, suggesting formation at relatively low-pressures. These zircon results provide evidence for 1.8 – 1.9 Ga metamorphism, possibly due to variable crustal thickening during a late Paleoproterozoic orogenic event. However, garnet porphyroblasts and the matrix minerals yield a Sm-Nd isochron age of 333.5 ± 4.0 Ma, whereas metamorphic titanites record a U-Pb age of 345.4 ± 2.7 Ma. Considering the high closure temperatures of the garnet Sm-Nd (650 – 720℃) and titanite U-Pb (710 – 780℃) systems, our data suggest that the late Paleoproterozoic metamorphic mineral assemblages were likely completely re-equilibrated by a Paleozoic tectonothermal event and that the P-T path documented above is a result of Paleozoic orogeny. In addition, apatite U-Pb dating yields 236.6 ± 4.3 and 300 ± 10 Ma for samples from the southern and central Dunhuang area, respectively, indicating different cooling histories. Combining with previous results, we propose that the Paleozoic metamorphism occurred in a long-lived hot continental arc with slow cooling rates (0.3 – 5℃/Myr), rather than an subduction–accretionary complex. Our study highlights the decoupling of metamorphic zircon U-Pb ages and P-T paths, which has significant implications for P-T-t path studies in polyphase metamorphic terranes.

Early Paleozoic Cascadia-type active-margin evolution of the Dunhuang block (NW China): Geochemical and geochronological constraints

Abstract Origins of early Paleozoic metabasites (granulites and amphibolites) and their host metasedimentary rocks in the Dunhuang block, NW China, are addressed by new geochronological and geochemical data. The metabasites show back-arc basalt–like geochemical features, marked by high Zr/Nb ratios and Zr-Hf troughs, but they can be classified into two groups based on their dissimilar protolith ages and distinct Nd signatures. Most group I metabasaltic rocks were emplaced before 455 Ma and possess high Ba/Nb ratios (11.46–224), low (Nb/La)PM (0.10–0.71), and negative whole-rock εNd(t) values (−12.7 to −2.7), whereas group II rocks have protolith ages around 445 Ma, low Ba/Nb ratios (0.70–22.93), low (Nb/La)PM (0.78–1.51), and less evolved whole-rock Nd isotopic features (εNd[t]: −2.0 to +2.7). It is proposed that group I metabasites originated from an enriched lithospheric mantle, while group II metabasites were derived from the depleted asthenospheric mantle. The metasedimentary rocks received detritus mainly from the neighboring Cambrian magmatic arc, and they are compositionally similar to active-margin sediments. Metamorphic zircon U-Pb ages ranging 462–422 Ma from the investigated rocks together with prominent magmatism further suggest high-grade metamorphism prevailing during the Late Ordovician–early Silurian. Based on these data, a Cascadia-type evolution is proposed involving an Ordovician–early Silurian suprasubduction stretching of the Cambrian active continental margin, which culminated with mantle upwelling. Recent paleogeographic reconstructions support the evolution and assembly of interior Proto–Tethys-Ran oceanic and continental plates, including the Dunhuang block, between 510 and 440 Ma, followed by Panthalassan subduction beneath the Tarim–North China continental assemblage at 440–430 Ma.

Petrogenesis of early Paleozoic granitoids in the Dunhuang Block, NW China: Insights into magma evolution and crustal thickening

The occurrence of adakitic rocks can provide insight into the evolution of continental crust and deep geodynamic processes in subduction zones. In this paper, we investigated zircon U-Pb ages and Hf isotope, whole-rock major and trace elements, and whole-rock Sr-Nd-Pb isotopes of the Lianghu quartz diorites and Shigong monzogranites in the Dunhuang Block, to understand their petrogenesis and tectonic setting as well as the tectonic evolution of the Dunhuang Block in the early Paleozoic. LA-ICP-MS zircon U-Pb dating results suggest that the Lianghu quartz diorites and Shigong monzogranites were emplaced at ca. 455 ± 3 Ma and 431 ± 3 Ma, respectively. The Ordovician quartz diorites exhibit relatively high Sr (734–904 ppm) contents, negative-to-negligible Eu anomalies (Eu/Eu* = 0.87–0.99), high Sr/Y (52.5–124) ratios, but low Yb (0.68–1.32 ppm) and Y (7.27–14.0 ppm) contents. Moreover, they exhibit relatively high Mg# (52.0–54.7) values, Cr (23.1–88.0 ppm) and Ni (15.0–53.3 ppm) contents, similar to those of adakitic rocks originating from subducted oceanic crust. The quartz diorites display low initial 87Sr/86Sr (0.703464–0.704783), negative εNd(t) of −3.2 to −1.3, and positive εHf(t) of +3.8 to +8.0, with two-stage model ages for the Nd and Hf systems of 1.12 to 1.09 Ga and 1.17 to 0.91 Ga, respectively, indicative of a heterogeneous magma sources. They have relatively high radiogenic 206Pb/204Pb(t) (18.219–19.066), 207Pb/204Pb(t) (15.616–15.669), and 208Pb/204Pb(t) (39.042–39.198), signifying that some subduction-related materials (e.g., ocean island basalts) may be involved as sources. These characteristics indicate that the Lianghu quartz diorites were derived from the partial melting of subducted oceanic slabs (sediments + fluids), and that they underwent subsequent interaction with peridotite in the mantle wedge, with limited assimilation of crustal materials during magma ascent. The Silurian monzogranites contain high Sr content (655–674 ppm) but very low contents of Y (2.49–4.19 ppm) and Yb (0.24–0.37 ppm), yielding high Sr/Y ratios (161–263); those values, together with low MgO (0.30–0.41 wt.%), Mg# (40.6–41.4), Cr (2.39–3.28 ppm), and Ni (2.19–2.95 ppm) contents, resemble those of thickened lower crust-derived adakitic rocks. They show εHf(t) and εNd(t) values of −2.5 to +3.0 and −3.6, with the respective two-stage model ages of 1.55 to 1.20 Ga and 1.32 Ga. Their low Pb values, 206Pb/204Pb(t) = 18.184, 207Pb/204Pb(t) = 15.595, and 208Pb/204Pb(t) = 37.717, also suggest addition of minor mantle-like melts. These geochemical features indicate that the parent magmas of the monzogranites originated either from the melting of heterogeneous crustal sources, or through mixing of thickened juvenile crust, Mesoproterozoic crust materials, and minor mantel-derived materials in the stability field of garnet-bearing amphibolite. Given the coeval regional magmatism and tectonic setting, we propose that the continental crust thickening of the Dunhuang Block began roughly in the late Ordovician (~35 km) and culminated in the early Silurian (~50–55 km), as evidenced by the La/Yb ratios of quartz diorites (12.2–25.2) and monzogranites (57.6–60.9). The corresponding transition of the tectonic regime from subduction to collisional setting occurred at ~440 Ma. The Dunhuang Block could undergo thickening processes associated with underplating of mantle-derived magmas during subduction of the southern Paleo-Asian Ocean, and subsequently collided with the Shibanshan terrane (a Paleozoic continental arc) along the southern Beishan orogen at ca. 440–430 Ma. Our results show that the Dunhuang Block represents a continental arc in the early Paleozoic, and was involved in the Paleozoic orogenesis of the Central Asian Orogenic Belt.

Origins of the meta‐mafic rocks in the southern Dunhuang Block (NW China): Implication for tectonic framework of the southernmost Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogenic collages in the world with prominent juvenile crust addition, where the southernmost margin of the CAOB located is one hot topic in the international studies. The Dunhuang Block located to the south of the Beishan Orogen is a key position for outlining the tectonic framework of the south margin of the CAOB. However, at present, the tectonic attribution of the Dunhuang Block is still controversial. In this paper, we focus on the origins and protoliths of the meta‐mafic rocks from the Hongliuxia complex in the southern Dunhuang Block for determining the tectonic attribution of the Dunhuang Block. In the field, the meta‐mafic rocks are mainly exposed as tectonic blocks within the metasedimentary rocks displaying the “block‐in‐matrix” fabrics or as tectonic slices juxtaposed by faults. Geochemically, the meta‐mafic rocks could be divided into three groups based on the rare earth elements (REE) and trace elements: the group 1 samples are depleted in the LREE relative to HREE and show flat HREE patterns, similar to the mid‐oceanic ridge basalt; the group 2 samples show flat REE patterns without obvious differentiation between the LREE and HREE, like the oceanic plateau basalt; the group 3 samples are enriched in LREE relative to LREE, and have negative Nb, Ta, and Ti anomalies, similar to the island arc basalt. Conclusively, the protoliths of the meta‐mafic rocks dominantly belong to the oceanic plate components. They were metamorphosed during Early Silurian to Middle Devonian in different depths of the subduction zone documented in previous studies. So, we suggest the Hongliuxia complex is an exhumed Palaeozoic subduction–accretion complex (SAC) formed during the oceanic plate subduction. This result confirms the Palaeozoic subduction event occurred in the Dunhuang region and indicates that the Dunhuang Block is a Palaeozoic Orogen rather than a part of Precambrian continent block. Considering the regional geology context, we speculate the Dunhuang Orogen belongs to the southernmost margin of the CAOB.

Paleozoic tectonics of southern Central Asian Orogenic Belt: insight from the interaction between the south Beishan Orogenic Belt and Dunhuang block

Paleozoic tectonics of southern Central Asian Orogenic Belt: insight from the interaction between the south Beishan Orogenic Belt and Dunhuang block

Geochronology, geochemistry, and Sr-Nd-Pb-Hf isotopes of the late Paleozoic Dongshuigou composite pluton in the Dunhuang Block, southernmost Central Asian Orogen: Petrogenesis and implications for crustal growth

Late Paleozoic granitoid rocks hold critical information regarding continental crustal growth and evolution of the Dunhuang Block at the southernmost margin of the Central Asian Orogenic Belt (CAOB). Here, we report new zircon U-Pb ages and Hf isotope data, as well as whole-rock geochemistry and Sr-Nd-Pb isotopic compositions for Devonian granitoid rocks and dacite porphyry in the Dongshuigou area of the Dunhuang Block, NW China. Petrographic investigation suggests that the Dongshuigou composite pluton, mainly consisting of quartz diorite, granodiorite, and biotite granite, was emplaced in the middle Devonian (ca. 390–380 Ma), whereas the dacite porphyry intruded into the granodiorite in the late Devonian (ca. 367 ± 4 Ma). The quartz diorite and granodiorite are similarly sodic, middle-K calc-alkaline, high Mg# (48.5–49.8) and Cr-Ni contents, enriched in LREE and LIIE (e.g., Rb, Ba, and P), and depleted in HFSE (e.g., Na and Ta), indicative of typically arc-like geochemical signatures. The quartz diorite has positive εNd(t) (+0.38), variable εHf(t) values (+3.12 to +10.7), juvenile Nd (1.04 Ga), and Hf model ages of 0.83 to 1.15 Ga. In comparison, the granodiorite displays negative εNd(t) values (−0.73) closed to the chondrite line, positive εHf(t) values (+3.75 to +8.36), juvenile Nd (1.12 Ga), and Hf model ages of 0.83 to 1.12 Ga. Both plutonic rocks show low initial Pb isotopes similar to those of typical lower crust-derived rocks. Our data indicate that the diorites and granodiorites were derived from partial melting of the juvenile lower crust with addition of minor mantle materials. The high-K, calc-alkaline dacite porphyry exhibits relatively high SiO2 and K2O contents, moderate Mg# (45.2–46.2) values, enrichment in Rb, U, Pb, and K, depletion in Nb, Ta, and Ti, and low Sr and Y abundance. These geochemical characteristics are similar to those of lower crust-derived arc-related rocks, including the development of abundant inherited zircons, implying that the dacite porphyry was likely generated by a mixed source of the mafic lower crust and minor metasomatized mantle materials. The biotite granites are characterized by high Sr contents (481–542 ppm) and Sr/Y ratios (76.0–98.4), low Y (5.37–6.51 ppm) and Yb (0.50–0.56 ppm) contents, and high Mg# values (55–56) and Cr-Ni contents, showing geochemical affinities of adakitic rocks. They have positive εNd(t) (+2.97 to +3.00), variable εHf(t) values (−6.05 to +12.4; mostly >0) with Nd and average Hf model ages of 0.82 Ga and 0.92 Ga, respectively. The biotite granites exhibit low initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.336–18.339, 15.622–15.622, and 38.478–39.510, respectively, indicating that some subduction-related materials, such as mature arc primitive or ocean island volcanic rocks could have been added to the magma source. The biotite granites were likely derived from partial melting of subducted oceanic crust, which was modified by interactions with mantle wedge, with the addition of minor Mesoproterozoic materials during magma ascent. By synthesizing with the regional geology, we suggest that the middle Devonian granitoids and the late Devonian dacite porphyry in the Dongshuigou are arc/subduction-related rocks that formed in a continental arc setting. Taken together, Devonian arc-related magmatic events in the Dunhuang Block indicate significant crustal growth that was accompanied with minor reworking of ancient lower crust.

Reworked early Precambrian basement of the Dunhuang Block (NW China) as revealed by titanite U–Pb dating

The Dunhuang Block in the Central Asian Orogenic Belt (CAOB) consists of Archean to Paleoproterozoic crystalline basement, along with Paleozoic complexes that are related to the evolution of the CAOB. A current main controversy concerns whether the whole Dunhuang Block has experienced the Paleozoic tectonothermal events. Titanite, a common U–bearing accessory mineral in igneous and metamorphic rocks, is susceptible to magmatic and metamorphic processes, and thus tends to record relatively late tectonothermal events for metamorphic rocks. In this study, we conducted LA–ICPMS U−Pb dating on titanites from the Archean to Paleoproterozoic rocks in the Dunhuang Block. The titanites are likely metamorphic in origin and yielded U–Pb ages of 409−309 Ma. These ages can be divided into two major groups at 409−390 Ma and 354−309 Ma, corresponding to an early-stage retrograde of high–grade metamorphism and a later-stage magmatism, respectively. Overall data and regional geology indicate that the whole Dunhuang Block has been intensively reworked by the Paleozoic tectonothermal events, leading to metamorphic and magmatic overprints of the early Precambrian basement rocks during the evolution of the CAOB in the Paleozoic. Therefore, the Dunhuang Block should not be considered as an independent Paleozoic orogenic belt. More importantly, our study demonstrates that titanite U–Pb dating can be used as a potential geochronologic tool to reveal late tectonothermal events of the reworked Precambrian metamorphic terranes.

Paleoproterozoic multiple magmatic-metamorphic events in the Dunhuang Block, eastern Tarim Craton: Implications for assembly of the Columbia supercontinent

The evolution of the Dunhuang Block associated with the assembly of the Columbia supercontinent is poorly constrained. The geochemical composition and temporal evolution of magmatic and metamorphic rocks may hold key information regarding critical geodynamic processes that occurred during the evolution of the Dunhuang Block. In this paper, we present a systematic petrographic, geochemical, and zircon U-Pb and Hf isotopic investigation on tonalitic gneiss, carbonatite, granitic dyke, mafic granulite and A-type granite in the Dunhuang Block. The results suggest that tonalitic gneiss was derived from partial melting of a thickened oceanic plateau crust or island-arc crust at ~3.67 Ga and then it recorded an important subduction-collision orogenesis in the Paleoproterozoic (~2.02 Ga to ~1.96 Ga). The ~1.94 Ga carbonatite and ~1.85 Ga granitic dyke were formed by partial melting of marble and Meso- to Neoarchean basement rocks, respectively. Both of them were formed in post-collisional setting. The ~1.82 Ga mafic granulite recorded high-pressure granulite facies metamorphism followed by a medium–low pressure granulite facies metamorphic overprint, suggesting a clockwise P-T path and implying an environment of collisional orogenesis. The ~1.76 Ga A-type granite resulted from the reworked Neoarchean crust and was most likely related to a post-collisional episode. Combined with previous studies, we identify two Paleoproterozoic orogenic belts of the Dunhuang Block: the ~2.00–1.96 Ga South Orogen and ~1.85–1.80 Ga North Orogen. According to the coupling of these two orogenies with the Columbia orogeny, we conclude that the Dunhuang Block was associated with the assembly of the Columbia supercontinent.